1. Field of the Invention
The present invention relates to a structure of a claw-pole type stepping motor, and particularly to a structure thereof for achieving reduction in dimension and offering technical advantages.
2. Description of the Related Art
As various electronic devices are increasingly requested to be downsized, motors incorporated in the devices are also requested to be downsized. In a conventional claw-pole type stepping motor with an inner rotor, A-phase and B-phase driving coils are typically disposed so as to surround the outer circumference of a rotor. In such a claw-pole type stepping motor, the diameter of a rotor magnet is restricted by the inner diameter of a stator, so a motor with a smaller dimension is forced to have a rotor magnet with a smaller diameter thus resulting in significant deterioration in motor characteristic. This makes it difficult for a claw-pole type stepping motor with a small radial dimension to ensure performance characteristic required for poisoning control. In such a case, a brushless DC motor equipped with an encoder as a position detector must be used in place of a claw-pole type stepping motor, which inevitably pushes up production cost.
Under the aforementioned circumstances, Japanese Patent Application Laid-Open No. 2003-009497 discloses a motor structured such that driving coils are disposed so as to axially sandwich a rotor magnet. This motor structure helps reduction of its radial dimension, but since the motor is entirely covered by a case, the motor size has to be increased for the thickness of the case in all directional dimensions including the radial dimension. In order to reduce the radial dimension of this motor, the thickness of the case must be decreased. A case with a small thickness is technically difficult to fabricate, and also is inferior in mechanical strength, thus effort in reducing the thickness of the case has its limit. The case may be eliminated by, for example, resin-molding a stator for an integral structure, and such a solution is mentioned in the aforementioned Japanese Patent Application Laid-Open No. 2003-009497 (refer to Paragraph [0004]). However, the solution is negatively discussed therein, because when bobbins having respective exciting coils wound therearound are resin-molded integrally with yokes, it happens occasionally that the terminal wires of coils are broken due to molding pressure. Specifically, resin injected for molding gets in direct touch with the surface of magnet wires thereby deteriorating the coating of the magnet wires or even deforming and breaking the magnet wires.